Many physicists say space is quantized. Or, as Smolin says, it has an atomic structure. If so, there’s a smallest piece of space, a piece that cannot be divided, inside which nothing penetrates.
As Smolin says this spawns a Problem: How the pieces fit together. Observations show that the result is very smooth. At the smallest scale, space pieces must fit neatly and this neatness must extend in exact fashion everywhere. How could a single structure spread across the vastness of the universe? Smolin’s point is not just that the fit must be exact. It’s not just that they must line up like atoms in a perfect crystal, though he says this too. It’s that right across the universe―in places that know nothing of each other―they must line up the same way. It’s a bit like the Horizon Problem. It comes down to the speed of light. If no message can go faster, how do far-flung parts of space discern the structure they must have?